Field of the Invention
The present invention relates to an air-conditioning apparatus in which a plurality of indoor units are connected in parallel to one outdoor unit by refrigerant pipes, and more specifically, to an air-conditioning apparatus in which outdoor heat exchangers are efficiently used while the refrigerant circulation amount is prevented from being insufficient when the cooling operation is performed under a condition where the outside air temperature is low.
Related Art
Conventionally, a so-called cooling and heating free air-conditioning apparatus is known in which a plurality of indoor units are connected in parallel to one outdoor unit by refrigerant pipes and each indoor unit can selectively perform a cooling operation and a heating operation. In this air-conditioning apparatus, for example, a plurality of indoor units are placed in different rooms, and it is possible to perform the cooling operation by an indoor unit while performing the heating operation by another indoor unit.
In this type of air-conditioning apparatus, the following are alternately connected by refrigerant pipes such as high pressure gas pipes, low pressure gas pipes and fluid pipes: an outdoor unit having a compressor, an outdoor heat exchanger, a flow path switching valve such as a three-way valve or a four-way valve, and an outdoor expansion valve; a plurality of indoor units having an indoor heat exchanger and an indoor expansion valve; and a plurality of flow dividing units provided so as to correspond to the indoor units and switching the flow direction of the refrigerant flowing in the indoor units.
In such an air-conditioning apparatus, when all the indoor units are performing the cooling operation or when the load required by the indoor units performing the cooling operation is heavier than that required by the indoor units performing the heating operation, the outdoor heat exchanger is used as a condenser. When all the indoor units are performing the heating operation or when the load required by the indoor units performing the heating operation is heavier than that required by the indoor units performing the cooling operation, the outdoor heat exchanger is used as an evaporator.
In an office building, a commercial facility or the like where such an air-conditioning apparatus is installed, when there are rooms in which an apparatus generating heat is placed such as a server room where a computer server is placed and a test room where a test apparatus generating a large amount of heat is placed, the indoor units placed in these rooms perform the cooling operation irrespective of the season to maintain the room temperature at a predetermined temperature, thereby preventing the apparatus generating heat from being adversely affected by the high temperature.
However, in winter, when the outdoor heat exchanger is used as a condenser under a condition where the outside air temperature is extremely low, for example, equal to or lower than −10 degrees C., there is a possibility that the heat exchange between the refrigerant and the outside air at the outdoor heat exchanger is performed more than necessary to reduce the pressure of the refrigerant sent to the indoor units. If the pressure of the refrigerant is reduced, the refrigerant circulation amount in the refrigerant circuit of the air-conditioning apparatus is reduced, so that there is a possibility that the evaporating pressure at the indoor heat exchangers of the indoor units performing the cooling operation is reduced to reduce cooling ability.
To solve such a problem, an air-conditioning apparatus has been proposed in which the outdoor heat exchanger of the outdoor unit is divided into a plurality of units, an outdoor expansion valve is connected to each unit and the number of outdoor heat exchangers used is determined according to the detected outside air temperature and refrigerant temperature (for example, refer to JP-A-2004-3691 (pages 4 to 6, FIG. 1)). In this air-conditioning apparatus, when the outdoor heat exchangers are used as condensers under a condition where the outside air temperature is extremely low, by fully closing the outdoor expansion valves connected to the outdoor heat exchangers other than the outdoor heat exchangers used, the outdoor heat exchangers corresponding to the fully closed outdoor expansion valves are made unused to thereby reduce the number of outdoor heat exchangers used. By doing this, the heat exchange between the refrigerant and the outside air at the outdoor heat exchangers can be prevented from being performed more than necessary to reduce the pressure of the refrigerant sent to the indoor units, so that the evaporating pressure at the indoor heat exchangers of the indoor units performing the cooling operation can be prevented from being reduced by the reduction in refrigerant circulation amount to reduce the cooling ability.